class Book
  include Mongoid::Document
  embeds_many :pages
  field :title
end
class Page
  include Mongoid::Document
  embedded_in :book
  field :text
end

b = Book.create(:title => "Cloud Atlas")
b.pages.update_all(:text => "Sonmi")

# NoMethodError: undefined method `find' for nil:NilClass
#   from /Users/jonathan/.rvm/gems/ruby-1.9.3-p194/bundler/gems/mongoid-9aa7e2975877/lib/mongoid/contextual/memory.rb:269:in `update_documents'
#   from /Users/jonathan/.rvm/gems/ruby-1.9.3-p194/bundler/gems/mongoid-9aa7e2975877/lib/mongoid/contextual/memory.rb:245:in `update_all'
#   from /Users/jonathan/.rvm/gems/ruby-1.9.3-p194/bundler/gems/mongoid-9aa7e2975877/lib/mongoid/contextual.rb:19:in `update_all'
#   from /Users/jonathan/.rvm/gems/ruby-1.9.3-p194/bundler/gems/mongoid-9aa7e2975877/lib/mongoid/relations/embedded/many.rb:412:in `block in method_missing'
#   from /Users/jonathan/.rvm/gems/ruby-1.9.3-p194/bundler/gems/mongoid-9aa7e2975877/lib/mongoid/scoping.rb:190:in `with_scope'
#   from /Users/jonathan/.rvm/gems/ruby-1.9.3-p194/bundler/gems/mongoid-9aa7e2975877/lib/mongoid/relations/embedded/many.rb:411:in `method_missing'
#   from (irb):13
#   from /Users/jonathan/.rvm/gems/ruby-1.9.3-p194/gems/railties-3.2.8/lib/rails/commands/console.rb:47:in `start'
#   from /Users/jonathan/.rvm/gems/ruby-1.9.3-p194/gems/railties-3.2.8/lib/rails/commands/console.rb:8:in `start'
#   from /Users/jonathan/.rvm/gems/ruby-1.9.3-p194/gems/railties-3.2.8/lib/rails/commands.rb:41:in `<top (required)>'
#   from script/rails:6:in `require'
#   from script/rails:6:in `<main>'

b.pages.create()
# => #<Page _id: 5060daebdd365a2c52000002, text: nil> 

b.pages.update_all(:text => "Sonmi")
#  => {"updatedExisting"=>true, "n"=>1, "connectionId"=>89, "err"=>nil, "ok"=>1.0}